1. Field of the Invention
The present invention relates to a superconducting cable conductor employing an oxide superconductor, and more particularly, it relates to a flexible conductor which is applicable to a superconducting cable.
2. Description of the Background Art
An oxide superconductor which enters a superconducting state at the liquid nitrogen temperature is expected for application to a superconducting cable with a cooling medium of liquid nitrogen. When such application is implemented, it will be possible to simultaneously attain simplification of a thermal protection system and reduction of a cooling cost in relation to a metal superconducting cable which requires high-priced liquid helium at present.
A superconducting cable must be capable of transmitting a heavy current with low energy loss in a compact conductor. Power transmission is generally made through an alternating current, and a superconductor employed under an alternating current is inevitably accompanied by energy loss which is generically called ac loss. The ac loss such as hysteresis loss, coupling loss or eddy current loss depends on the critical current density (Jc) of the superconductor, sizes of filaments, the structure of the conductor, and the like.
Various types of superconducting cables have heretofore been manufactured through metal superconductors, with study of structures for reducing ac loss. For example, Japanese Patent Publication No. 6-36329 (1994) discloses a superconducting conductor which comprises a normal conductor, and composite multifilamentary superconductors which are spirally wound along the outer periphery of the normal conductor. The conductor disclosed in this gazette is formed by clockwisely and anticlockwisely wound layers of the composite multifilamentary superconductors, which are alternately superposed with each other. The directions for winding the conductors are varied every layer for reducing magnetic fields generated in the conductors, thereby reducing impedance and increasing current carrying capacity thereof. This gazette also proposes provision of a high-resistance or insulating layer between the layers, in order to reduce ac loss.
When a cable conductor is formed by an oxide superconductor, a technique employed in a metal superconductor cannot be applied as such. An oxide superconductor, i.e., a ceramics superconductor, is fragile and weak against mechanical strain as compared with a metal superconductor. For example, Japanese Patent Publication No. 6-36329 (1994) discloses a technique of spirally winding the superconductors around the normal conductor so that the winding pitch is equal to the diameter of each superconductor. However, when a recently developed superconducting wire comprising an oxide superconductor which is covered with a silver sheath is wound at such a short pitch, for example, there is a high possibility that the oxide superconductor is broken to disable current feeding. When an oxide superconducting wire is extremely bent, its critical current may be remarkably reduced. In manufacturing of a cable conductor, therefore, an important subject is how to arrange the oxide superconductor.
Further, the cable conductor must be flexible to some extent, for facilitating handling. It is also an important subject how to manufacture a flexible cable conductor through a hard and fragile oxide superconductor.
In addition, ac loss inevitably follows a superconductor which is employed under an alternating current, as described above. Thus, it also remains as an important subject how to reduce ac loss in manufacturing of a cable conductor through an oxide superconducting wire.